Ground fault location system and ground fault detector therefor

ABSTRACT

A ground fault location system is used in a multi-phase ungrounded or high-impedance grounded power network. A signal generator is coupled to the network at a first location and generates for each network phase an individual non-DC voltage signal between such phase and ground. A ground fault detector is coupled to the network at a second location and has a summing device and an annunciator. The summing device is coupled to all of the phases of the network at such second location, sums any current passing therethrough, and produces a sum signal. The annunciator receives the sum signal and provides an indication when such signal is non-zero. Each phase of the network at the second location has a distribution current passing therethrough, the sum thereof normally being substantially zero and resulting in a substantially zero sum signal and the lack of an indication from the annunciator based on such distribution currents. When the second location is on a path between the first location and a ground fault, the individual voltage signal on at least one of the phases generates a fault current thereon through such path and results in a non-zero sum signal and an indication from the annunciator. When the second location is not on such path, none of the individual voltage signals generates a fault current on any phase through such path, resulting in a substantially zero sum signal and the lack of an indication from the annunciator.

FIELD OF THE INVENTION

The present invention relates to a ground fault detector system fordetecting a ground fault in a multi-phase ungrounded or high impedancegrounded electrical power distribution network, and a ground faultdetector used therewith. More particularly, the present inventionrelates to such a ground fault location system where the detectordetects an artificial injected signal.

BACKGROUND OF THE INVENTION

In a power distribution network, one or more distributed phases canbecome faulted to ground. Reasons for such a ground fault are numerousand known, and include damage to electrical insulation and theconnection of a defective device to the power network, among others.

In high-impedance grounded networks and ungrounded networks (i.e., anynetwork where there is no significant current path to ground),phase-to-ground faults produce relatively insignificant values of faultcurrent. Typically, in small isolated-neutral industrial installations,the ground-fault current for a phase fault may be well under an amp.Correspondingly, in a large plant containing miles of cable, suchcurrent may be no more than 20 amps. Such currents usually are not ofsufficient magnitude for the operation of zero-sequence over-currentrelays, ground fault relays, fault-sensing circuit breakers, fuses, andsimilar protective devices to locate and remove such faults. Suchprotective devices do not have the sensitivity necessary due to thecomplexity of the current flow pattern between the distributed cablecapacitance and the fault.

The primary indication of a ground fault in such a network is a shift inground potential with respect to phase voltage. Although such indicationcan be sensed and employed as an alerting device, such indication doesnot assist in determining the location of the ground fault, which may beanywhere within the network.

An accurate ground fault location system is an important tool that isuseful in minimizing the economic and maintenance impact of faults onungrounded and high-impedance grounded networks. The benefits ofaccurate fault location include reduced switching operations required toisolate a faulted feeder section or load; reduced search efforts tolocate a fault, thereby facilitating faster repair time; and greaterlikelihood that a detected fault will be addressed before a second faultoccurs in the network. Accordingly, a need exists for an accurate groundfault location system and a detector therefor.

SUMMARY OF THE INVENTION

The present invention satisfies the aforementioned need by providing aground fault location system and a detector therefor. The ground faultlocation system is used in a multi-phase ungrounded or high-impedancegrounded electrical power distribution network. The system comprises asignal generator and the ground fault detector. The signal generator iscoupled to the network at a first particular network location andgenerates for each phase of the network an individual non-DC groundfault detection voltage signal between such phase and ground.

The ground fault detector is coupled to the network at a secondparticular network location separate from the first location andcomprises a summing device and an annunciator. The summing device iscoupled to all of the phases of the network at such second location, andsums any current passing therethrough and produces at an output thereofa sum signal representative of such summed current. The annunciator iscoupled to the output of the summing device to receive the sum signal,and provides an indication when the sum signal is non-zero.

Each phase of the network at the second location has a distributioncurrent passing therethrough and the sum of the distribution currentfrom each phase is normally substantially zero. The zero-sumdistribution current at the second location results in a substantiallyzero sum signal and the annunciator does not provide the indicationbased on such distribution currents.

When the second location is on a path between the first location and aground fault, the individual voltage signal on at least one of thephases generates a fault current on such phase through such path. Thegenerated fault current results in a non-zero sum signal and theannunciator provides the indication.

When the second location is not on a path between the first location anda ground fault, none of the individual voltage signals on any of thephases generates a fault current on any phase through such path. Thelack of a generated fault current results in a substantially zero sumsignal and the annunciator does not provide the indication.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments of the present invention, will be betterunderstood when read in connection with the appended drawings. For thepurpose of illustrating the invention, there are shown in the drawingsembodiments which are presently preferred. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown. In the drawings:

FIG. 1 is a schematic diagram of a ground fault location system for amultiphase ungrounded or high-impedance grounded electrical powerdistribution network, in accordance with a preferred embodiment of thepresent invention;

FIG. 2 is a schematic diagram showing aspects of the system of FIG. 1 inmore detail;

FIG. 3 is a schematic diagram of a ground fault detector employed inconnection with the system of FIGS. 1 and 2 in accordance with apreferred embodiment of the present invention;

FIG. 4 is a schematic diagram showing aspects of the detector of FIG. 3in more detail; and

FIG. 5 is a timing diagram explaining the operation of the system anddetector shown in FIGS. 1-4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Certain terminology may be used in the following description forconvenience only and is not considered to be limiting. The words "left","right", "upper", and "lower" designate directions in the drawings towhich reference is made. Similarly, the words "inwardly" and "outwardly"are directions toward and away from, respectively, the geometric centerof the referenced object. The terminology includes the words abovespecifically mentioned, derivatives thereof, and words of similarimport.

Referring to the drawings in detail, wherein like numerals are used toindicate like elements throughout, there is shown in FIG. 1 a groundfault location system 10 for a multi-phase ungrounded or high-impedancegrounded electrical power distribution network 11 in accordance with apreferred embodiment of the present invention. As seen, the network 11is a branching network including a main branch, feeders, drops, etc(hereinafter `branches 12`). However, the network 11 may be any othertype of network without departing from the spirit and scope of thepresent invention. For example, the network 11 may be a non-branchingnetwork, a circular network, a multiple path network (having multiplepaths between a power source and a load), a multiple power sourcenetwork, etc. Presumably, one or more loads 18 are attached to thenetwork 11 at branches 12 thereof.

As seen in FIG. 1, the ground fault location system 10 of the presentinvention includes a signal generator 14 and one or more ground faultdetectors 16 positioned at locations on various of the branches 12 ofthe network 11. In the ungrounded or high-impedance grounded network 11,a ground fault does not complete a viable electric circuit. Accordingly,the system 10 of the present invention completes such viable circuit bycoupling the signal generator 14 to ground and having such generator 14produce a controlled fault voltage signal between each phase of thenetwork 11 and ground (i.e., between a first phase and ground, between asecond phase and ground, etc.).

In particular, the signal generator 14 is coupled to the network 11 at afirst particular network location and generates for each phase of thenetwork 11 an individual non-DC ground fault detection voltage signalbetween such phase and ground. Each ground fault detector 16 is coupledto the network 11 at a respective second particular network locationseparate from the first location to sense whether fault current based onthe fault voltage is flowing at the respective second location from thesignal generator 14 toward a ground fault.

As should be understood, such fault current will flow only on thosepaths which complete the fault circuit. Accordingly, each detector 16senses whether fault current is flowing at its respective secondlocation, and provides an annunciation if fault current is in fact soflowing. In particular, fault current sensed at a particular secondlocation by a respective detector 16 signifies that such particularsecond location is on a path to a ground fault. Correspondingly, thefailure to sense fault current at a particular second location by arespective detector 16 signifies that such particular second location isnot on a path to a ground fault.

By judicious placement of the detectors 16, the path to a ground faultcan be quickly defined. However, the system may be operated with asingle detector 16 without departing from the spirit and scope of thepresent invention. Preferably, if only a single detector 16 is employed,such detector is movable by a technician or other appropriate personnelamong multiple second locations on the network 11. In such a case, thelone detector 16 will sense whether fault current is present at eachparticular second location in seriatim, and intelligent movement of suchlone detector 16 by such technician can locate a ground fault relativelyquickly.

Preferably, the detectors 16 are permanently or semi-permanentlypositioned/located throughout the network 11 anywhere where it isconvenient to determining the location of a ground fault. On branches12, such detectors 16 are preferably located at regular intervals.Detectors 16 are also preferably located at branching points in thenetwork 11. Accordingly, where a first detector 16 is annunciating and asecond detector 16 downstream from the first detector 16 is notannunciating, the ground fault is either between such first and seconddetectors 16, or is downstream from a branching point between the firstand second detectors 16. Put another way, an annunciating detector 16indicates that a ground fault is downstream therefrom. For example, adetector 16 on a branch 12 to a load 18 will annunciate if such load 18is the cause of a ground fault.

Preferably, the signal generator 14 is operated in response to thedetection of a ground fault, where such ground fault is detected bymonitoring shifts in ground potential with respect to each phase.Typically, such monitoring function is performed by an appropriatelyconfigured protective relay, fault detector, or the like (not shown).The positive detection of a ground fault thereby is then appropriatelyreported to a technician or other appropriate personnel. Alternatively,in an automated system, such positive detection automatically triggersthe signal generator 14 to begin operating. Any appropriate manual orautomatic operation of the signal generator 14 based on a positivedetection of a ground fault may be employed without departing from thespirit and scope of the present invention.

The signal generator 14 is located at a convenient location in thedistribution network 11, such as near the power supplying transformer 22supplying power to the network 11, as is shown in FIGS. 1 and 2.However, the signal generator 14 may be located at most any convenientlocation within the network 11 without departing from the spirit andscope of the present invention. Accordingly, the term `downstream` asused above means downstream with respect to the flow of fault currentfrom the signal generator 14, and not necessarily downstream from thepower supplying transformer 22.

Referring now to FIG. 2, it is seen that the signal generator 14 may becoupled to each phase of the network 11 by way of a transformer 20 suchas the delta-wye transformer 20, as shown. Notably, such delta-wyetransformer 20 is coupled to ground at the neutral center point of the`wye` so as to provide the return path of the fault current through theground fault and back to the signal generator 14/transformer 20. Anyapparatus for coupling the signal generator 14 to each phase of the anetwork 11 may be employed without departing from the spirit and scopeof the present invention.

As also seen in FIG. 2, with the signal generator 14 and the transformer20 installed at the head of the main branch 12 of the network 11, andadjacent the power supplying transformer 22, the ground fault detectionvoltage signal generated by the signal generator 14 on each phasetravels from the head of the main branch 12 and down the network 11 to aground fault. With a detector 16 positioned at each drop 12 adjacent aload 18 being fed by such drop 12, as is shown, if the load 18 to theleft (i.e., the motor) is causing a ground fault, the detector 16 to theleft on the drop 12 to the left will annunciate. Likewise, if the load18 to the right is causing the fault, the detector 16 to the right onthe drop 12 to the right will annunciate. As should be understood,detectors 16 to the right will not annunciate if the load 18 to the leftis causing the fault, for the reason that the fault current will nottravel beyond the branch 12 to the load 18 to the left. That is, in sucha situation, the detector 16 to the right is not on the path of leastresistance to the ground fault associated with the load 18 to the left.

Referring now to FIG. 3, it is seen that each ground fault detector 16includes a current transformer 24 that is coupled to all of the phasesof the network 11 at the location thereof (i.e., at the second locationof such detector 16) such that all phases of the network 11 pass throughsuch current transformer 24. As a result, the current transformer 24sums any and all currents passing therethrough and transforms suchsummed current into a transformer voltage (i.e., a sum signal) appearingat an output 26 thereof. As should be understood, then, each detector 16is self-powered by the voltage produced by the current transformer 24 atthe output 26 thereof. As should also be understood, each currenttransformer 24 comprises an annular core with a series of windingswrapped therearound. The conductors that carry the phases pass throughthe annulus of the core, with the result being that such conductors actas the transformer primary, and the windings act as the transformersecondary. Any suitable current transformer may be employed as thecurrent transformer 24 without departing from the spirit and scope ofthe present invention.

Moreover, any suitable device or apparatus for summing the current atthe respective second location may be employed without departing fromthe spirit and scope of the present invention. For example, such deviceor apparatus may comprise individual transformers, one for each phase,and a summing device summing the respective outputs of suchtransformers. Alternatively, current-sensing resistors may beappropriately inserted into each phase at a respective second location,and a summing device may be employed to sum the respective voltage dropsacross such resistors.

Still referring to FIG. 3, each detector 16 also includes an annunciator28 coupled to the output 26 of the current transformer 24 to receive thetransformer voltage (or a modified form thereof) therefrom. Theannunciator 28 provides an indication when the transformer voltage atthe output 26 of the current transformer 24 is non-zero. Preferably, theannunciator 28 comprises a pair of LEDs, as is seen in FIG. 4.Accordingly, the annunciator provides a light indication from at leastone of the LEDs when the transformer voltage reaches a pre-determinedminimum turn-on voltage. If necessary, the annunciator 28 may alsoinclude a current limiting resistor R1, as seen in FIG. 4, to limit thecurrent through the LEDs. Preferably, the current limiting is selectedto limit such current below about 50 milliamps, and the LEDs areselected based on such current. Preferably, the pair of LEDs in theannunciator 28 are connected in anti-parallel, as shown. Accordingly,undesirable feedback that may arise from the non-linear aspects of theLEDs is minimized.

In addition or as an alternative to the LEDs shown in FIG. 4, theannunciator 28 may comprise other indicating/signaling devices,including a relay, a sound-emitting device, a communication signal, andthe like, without departing from the spirit and scope of the presentinvention. If a communication signal is provided from each of severaldifferent detectors 16 at several different respective second locations,such signals may be fed back to a central location for centralizedsignal processing and analysis.

As seen in FIGS. 3 and 4, each detector 16 may additionally include anappropriate frequency limiting filter 30 for frequency limiting thetransformer voltage prior to reception by the annunciator 28, and mayfurther include an appropriate voltage limiting filter 32 for voltagelimiting the transformer voltage prior to reception by the annunciator28. Any appropriate type of frequency limiting filter and/or voltagelimiting filter may be employed, or no filters at all may be employed,without departing from the spirit and scope of the present invention.

The frequency filtering component 30 may be any of several knownfilters, including an appropriate R-L-C filter (not shown). Suchcomponent 30 preferably limits the frequency of the transformer voltagesignal received by the annunciator 28 to a particular range associatedwith the voltage signal output by the signal generator 14. Accordingly,extraneous noise from a relatively noisy environment does not cause afalse annunciator indication. In a relatively quiet environment, thefrequency limiting component 30 may not be necessary at all.

The voltage limiting component 32 preferably prevents unusually hightransformer voltages at the output 26 of the current transformer 24 fromreaching the annunciator 28. Such a voltage limiting component 32 may bean electronic voltage regulator, a metal-oxide varistor (MOV) (notshown), or any other voltage-limiting technology. However, and as willbe described below, the voltage limiting component 32 may not in fact benecessary and if so can be removed from the detector 16.

The operation of the system 10 and detector 16 will now be discussedwith particular reference to the timing diagram shown in FIG. 5. As seenin FIG. 5, in a typical 3-phase network of the type shown in FIGS. 2-4,each of the 3 phases L1, L2, L3 has a generally sinusoidal distributioncurrent signal I_(L1), I_(L2), I_(L3) thereon as a distribution currentdestined for one or more (and perhaps many) loads 18. Normally, suchdistribution currents are balanced. Accordingly, the sum of thedistribution currents I_(L1), I_(L2), I_(L3) is normally substantiallyzero, as shown at (ΣI_(L1) -I_(L3)) in FIG. 5. Therefore, such zero-sumdistribution current ΣI_(L1) -I_(L3) when passing through the currenttransformer 24 of any particular detector 16 will produce asubstantially zero transformer voltage at the output 26 thereof. As aresult, the annunciator 28 receiving the transformer voltage therefromwill not annunciate based upon such zero-sum distribution current

As seen in FIG. 5, the individual fault voltage signals placed on eachof the lines L1-L3 by the signal generator 14 are FV_(L1) -FV_(L3),respectively. The fault voltage signals FV_(L1) -FV_(L3) may have anynon-DC frequency that is appropriate and feasible. For example, suchfrequency may be 40 hertz, 300 hertz or 1 kilohertz, etc. As seen inFIG. 5, such frequency is slightly higher than the frequency of thedistribution currents I_(L1) -I_(L3) (typically 50-60 hertz).

Regardless of any load 18 on the system 10, because the signal generator14 generating the fault voltages FV_(L1) -FV_(L3) is grounded, suchfault voltages FV_(L1) -FV_(L3) will not generate a current through anybranch 12 of the network 11 unless such branch 12 is on a path betweenthe signal generator 14 and a ground fault. Put simply, only the groundfault can provide a return for any of such fault voltage signals, andwithout such return, no fault current will flow in such path.

Still referring to FIG. 5, then, if as an example line L2 is groundfaulted, along the path between the signal generator 14 and the groundfault, it is seen that the fault current on lines L1 and L3 (FI_(L1),FI_(L3)) is substantially zero, while the fault current on line L₂(FI_(L2)) is not substantially zero and is in fact a current signal atthe same frequency as the fault voltage on line L2 (FV_(L2)).Accordingly, when the fault currents (or lack thereof) from Lines L1-L3are summed (ΣFI_(L1) -FI_(L3)), a non-zero-sum fault current is found toexist.

As should now be understood based on FIG. 5, a detector 16 placedanywhere on the network 11 will detect all three of the distributioncurrents I_(L1) -I_(L3) passing therethrough, and will normally sum suchdistribution currents I_(L1) -I_(L3) to a substantially zero value(ΣI_(L1) -I_(L3)). As a result, the zero-sum distribution current at thedetector 16 will result in a substantially zero transformer voltage atthe output 26 of the current transformer 24 of such detector 16.Accordingly, the annunciator 28 of such detector will not provide anindication based on such distribution currents I_(L1) -I_(L3).

However, if such detector 16 is on a path between the signal generator14 and a ground fault, the fault voltage FV_(L1) -FV_(L3) on at leastone of the phases will generate a fault current FI_(L1) -FI_(L3) on suchphase through such path, and the generated fault current (FI_(L2) inFIG. 5) will be summed by the current transformer 24 of such detector16, resulting in a non-zero transformer voltage at the output 26 of suchcurrent transformer 24. As should be appreciated, then, such non-zerotransformer voltage will cause the annunciator 28 to provide anindication, such as lit LEDs.

In contrast, if such detector 16 is not on a path between the signalgenerator 14 and a ground fault, none of the individual fault voltagesignals FV_(L1) -FV_(L3) on any of the phases will generate a faultcurrent FI_(L1) -FI_(L3) on any phase through such path. As should nowbe understood, the lack of any generated fault current will result in asubstantially zero transformer voltage on the output 26 of the currenttransformer 24 of such detector 16, and the annunciator 28 of suchdetector 16 will not provide any indication.

In a network 11 having a plurality of such detectors 16, then, severalof the detectors 16 will (hopefully) be on the path between the signalgenerator 14 and a ground fault, and the annunciators 28 of suchdetectors 16 will indicate such path. Accordingly, a maintenance personor technician searching for such ground fault need only follow the pathas outlined by such annunciators 28 to the vicinity of such groundfault. Once in such vicinity, visual inspection and other knowntechniques may be employed to physically locate and remedy such groundfault.

Preferably, the individual fault voltage signals generated by the signalgenerator 14 (FV_(L1) -FV_(L3)) are non-zero when summed in anycombination thereof. For example, and referring again to FIG. 5, it isseen that the fault voltages FV_(L1) -FV_(L3) on lines L1 through L3 areall in-phase. Accordingly, if any combination of lines L1-L3 are groundfaulted, the sum of the corresponding combination of fault currentsFI_(L1) -FI_(L3) resulting from such fault voltages FV_(L1) -FV_(L3)will result in a summed waveform of the same frequency and phase as anyof the fault currents FI_(L1) -FI_(L3), but perhaps a higher amplitudeif more than one line L1-L3 is faulted. The importance of theaforementioned non-zero summing of the individual voltage signals oneach phase is understood if the scenario is considered wherein a pair ofsuch individual fault voltage signals does sum to zero and the phases ofsuch pair of fault voltages are both ground faulted. In such a scenario,both fault voltages will produce fault currents, but such fault currentswill sum to zero and therefore not be detected by the detectors 16.

As was discussed above and referring to the currents I_(L1) -I_(L3)shown in FIG. 5, in normal operating conditions, the sum of distributioncurrents through the network 11 at any particular point (ΣI_(L1)-I_(L3)) will normally be substantially zero, or at least be no largerthan a minimal value. However, network fault conditions exist thatresult in large unsymmetrical (unbalanced) fault currents. Suchconditions may for example include phase-to-phase faults. In suchsituations, the summed network currents at a current transformer 24 of adetector 16 can be of magnitudes of 10,000 to 60,000 amps or higher.Such summed currents and the resulting transformer voltage at the output26 of such current transformer 24 will damage or destroy such detector16.

To protect each detector 16, then, from such fault conditions by way ofthe current transformer 24 coupling, a limiting technology must beemployed. As was discussed above, such limiting technology may be theaforementioned voltage limiting component 32 which could take the formof an electronic voltage regulator, a metal-oxide varistor (MOV) orother available technologies. However, in a preferred embodiment in thepresent invention, such limiting technology comprises the use of asaturating transformer core in the current transformer 24, where suchcore limits the transformer voltage that appears at the output 26 ofsuch current transformer 24. Unlike electronic voltage regulators orMOVs, a saturating transformer core does not reach a break downthreshold that will allow energy to pass on through from the output 26of the current transformer 24. However, the core must be designed towithstand the mechanical forces of maximum anticipated fault currents.

It is to be noted that the detectors 16 of the present invention willannunciate a path to the location of the phase-to-phase fault, at leastduring the period before a protective relay or the like detects andisolates such fault. In such an instance, the signal generator 14 neednot be operated since the unsymmetrical phase currents will power theannunciating detectors 16. Of course, after the fault has been isolated,an external source of fault-detection power is necessary to continue topower the annunciating detectors 16.

A saturating current transformer 24 operates according to basic currenttransformer principles, but has a very small amount of iron in thetransformer core. The iron content is just sufficient to induce aspecified voltage. At a higher current, corresponding to line fault, theoutput 26 of the saturating current transformer 24 will not be able todeliver much more than a designed normal current because the core willnot induce more voltage.

As primary current increases in a saturating core transformer, thesecondary current increases in proportion, and the flux necessary toinduce the necessary voltage to supply the burden will also increase inproportion. If the flux approaches the saturation value in the core, theexciting current will increase more than proportionally and thesecondary current accordingly will suffer and will cease to increaseproportionally. As the primary current increases still more, the flux inthe core will finally be limited by saturation during a considerablepart of the cycle. The secondary current, measured as a root meansquare, will still continue to increase but it will be very muchdistorted from a sinusoid wave shape. The phenomena is extremelycomplicated because the flux tends to leave the part of the core inwhich the density is highest, and complete its path in air.

In practice, high impedance loading or burden, connected to a currenttransformer, forces a proportional increase in the output voltage andthe magnetic flux. As the flux approaches the saturation value in thecore, the exciting current will increase very rapidly and the secondarycurrent will accordingly be noticeably decreased. If the burden isincreased to a value which would require a flux above the saturationvalue to induce the necessary voltage, the flux will not go beyondsaturation at the crest, but its rate of change as it goes through zeromay be just as high as it would be if it had gone to the full value. Inother words, the transformer performs normally during only part of thecycle. It is clear that the secondary current and voltage will be badlydistorted from the sinusoidal shape, with the voltage wave showingparticularly high peaks. The peaks are practically as high as they wouldhave been had the core not saturated.

If the voltage peak is too high, there is a danger of damaging theconnected annunciator 28. For this reason, it is desirable to usemagnetic materials having a rectangular hysteresis loop so that, atsaturation, the windings exhibit low inductance and minimal kickback.Preferably, the saturating core transformer 24 is formed from a steelsuch as SUPERMALLOY steel, or from any other steel with good saturation.

Preferably, the current transformer 24 provides a voltage at the output26 thereof of three to five volts at rated sensing current (typicallyone to five amps). A maximum voltage should not be more than one or twovolts higher than this level. A turn ratio of between about 1:50 andabout 1:150 may be employed to obtain sufficient sensitivity to lowsignal currents, although other turn ratios may be employed ascircumstances require without departing from the spirit and scope of thepresent invention.

The maximum number of turns on the core of the current transformer 24 isdependant on the cross-sectional area of the core. Commerciallyavailable cores having cross-sectional areas of about 150 mm² aresufficient. The resulting voltage at the output 26 of the currenttransformer 28 can be calculated according to the equation:

    V=4.444 B.sub.max A.sub.core ƒn,

where B_(max) is the induction level, which for SUPERMALLOY steel isabout 7.3 tesla. A_(core) is the cross-sectional area of the core. ƒ isthe system frequency. n is the number of turns on the core.

In the foregoing description it can be seen that the present inventioncomprises a new and useful ground fault location system and a groundfault detector therefor. It will be appreciated that changes could bemade to the embodiments described above without departing from the broadinventive concepts thereof. For example, rather than the signalgenerator 14--transformer 20 arrangement shown in FIG. 2 for generatingthe ground fault detection voltage signal on each phase of the network11, such signal may be generated: by individual signal generators 14,one coupled to each phase; by a single signal generator 14 selectivelycoupled to all the phases by appropriately controlled relays; or by asingle signal generator 14 coupled directly to the center point of azig-zag transformer, among other arrangements. It is understood,therefore, that this invention is not limited to the particularembodiments disclosed, but it is intended to cover modifications withinthe spirit and scope of the present invention as defined by the appendedclaims.

What is claimed:
 1. A ground fault detector for a multi-phase ungroundedor high impedance grounded electrical power distribution network,wherein a signal generator is coupled to the network at a firstparticular network location, the generator generating for each phase ofthe network an individual non-DC ground fault detection voltage signalbetween such phase and ground, the ground fault detector for beingcoupled to the network at a second particular network location separatefrom the first location and comprising:a summing device for beingcoupled to all of the phases of the network at such second location, thesumming device summing any current at such second location and producingat an output thereof a sum signal representative of such summed current;and an annunciator coupled to the output of the summing device forreceiving the sum signal, the annunciator for providing an indicationwhen the sum signal is non-zero; wherein when the detector is coupled tothe network and the second location is on a path between the firstlocation and a ground fault, the individual voltage signal on at leastone of the phases generates a fault current on such phase through suchpath, the generated fault current resulting in a non-zero sum signal andthe annunciator providing the indication.
 2. The detector of claim 1wherein the summing device is a current transformer for being coupled toall of the phases of the network at such second location such that allphases of the network pass through such current transformer, the currenttransformer summing any current passing therethrough and transformingsuch summed current into a transformer voltage, the transformer voltagebeing the sum signal.
 3. The detector of claim 2 wherein the currenttransformer is a saturating transformer having a saturation core thatlimits the transformer voltage that appears at the transformer output.4. The detector of claim 1:wherein when the detector is coupled to thenetwork, each phase of the network at the second location has adistribution current passing therethrough and the sum of thedistribution current from each phase is normally substantially zero, thezero-sum distribution current at the second location resulting in asubstantially zero sum signal and the annunciator not providing theindication based on such distribution currents; and wherein when thedetector is coupled to the network and the second location is not on apath between the first location and a ground fault, none of theindividual voltage signals on any of the phases generates a faultcurrent on any phase through such path, the lack of a generated faultcurrent resulting in a substantially zero sum signal and the annunciatornot providing the indication.
 5. The detector of claim 1 wherein theannunciator comprises a pair of LEDs connected in anti-parallel forreceiving the sum signal and generating a corresponding currenttherethrough.
 6. The detector of claim 5 wherein the annunciator furthercomprises a current limiting resistor for limiting the LED current. 7.The detector of claim 1 wherein the annunciator further comprises afrequency limiting filter for frequency-limiting the sum signal receivedby the annunciator.
 8. The detector of claim 1 wherein the annunciatorfurther comprises a voltage limiting filter for voltage-limiting the sumsignal received by the annunciator.